The present invention relates to a decorative, and especially a plate-shaped molded article, consisting of a core of a fiber-containing material, coated with a thermosetting synthetic resin, which is then heat-hardened and pressed, and finally a decorative layer is applied to the surface or surfaces of the core.
The invention relates also to a process for the preparation of a molded article of this type, wherein one or several layers of a fibrous material are arranged above each other, are provided with a thermosetting synthetic resin, and are pressed together at an elevated temperature to form the core of the molded article, whereby the synthetic resin is hardened, and the surface or surfaces of the core are provided with a decorative layer during the heat-treatment or subsequent to it.
The invention further relates to the use of the decorative, preferably plate-shaped molded article.
To produce the known decorative laminated sheets, generally kraft paper is first impregnated with a low molecular weight duroplastic resin and dried at 120.degree. to 160.degree. C., whereby the resin is partially hardened. A number of these pretreated papers are stacked upon one another and form the core of the laminated plate which is to be made. Onto one or both of the outer paper layers of the stack, usually a decorative layer of paper or possibly a covering paper sheet are placed, which usually are also treated in an appropriate manner with a duroplastic resin. The stack is hardened at a temperature of 120.degree. to 180.degree. C. under a pressure of at least 70 bar in a stationary press installation. In the course of this process, the resins flow and are hardened, whereby the individual layers of the core and the surface layers are shaped into a nonmelting, rigid cross-linked sealed product. The laminated plate obtained is used primarily in the furniture and construction industries.
This known laminated plate has the disadvantage of a relatively low dimensional stability under varying climatic conditions. With varying atmospheric humidities and changes in temperature, the laminated plate expands or shrinks in the longitudinal direction and particularly strongly transverse to the longitudinal direction. The originally flat laminated plate may warp upon absorbing moisture and attain a concavely curved surface, or else its surface may exhibit a structure corrugated in cross-section after the absorption of moisture.
This dimensional stability, which is inadequate for constructional applications, originates in the strucuture of the core of this known laminated plate. The kraft papers forming the core consist of fibers arranged up to 60 to 80% in the longitudinal direction and up to 40 to 20% in the transverse direction, with respect to the web of the paper. Consequently, the properties of these papers are different in these two directions. This orientation of the paper, which is particularly strong in the longitudinal direction of the web of paper, is further enhanced during the impregnation of the paper web with thermosetting resins dissolved in water or organic solvents and the subsequent drying, if the wet kraft paper is exposed to tensile stress in the longitudinal direction. This relatively strong orientation in the longitudinal direction has the effect that the resulting laminated plate based on kraft paper exhibits dimensional variations that differ in the longitudinal and transverse directions under varying climatic conditions.
Kraft papers are not homogeneous in relation to their thickness either. For manufacturing reasons, their top side has a lower density than their screen side. Complete homogeneity is for this reason fundamentally not achievable.
Furthermore, in the conventional process the complete impregnation and wetting of the individual paper fibers is difficult to control because of the different density of the webs of paper and because of the penetration properties for the resin solutions which vary for this reason. Fibers which are not coated with the resins may cause blister formation in the core when used in wet, humid or exterior applications.
Beyond this, as the result of the stratified configuration of the plate and the lack of homogeneity of the paper web, a relatively slight and partially uncertain reinforcement between the individual core layers exists, whereby the transverse tensile strength may be rendered relatively low and failures may occur under transverse stress. In the course of long-term fatigue exposures in the outside atmosphere or in atmospheric test cabinets or in the case of weathering tests, this inhomogeneity may lead to the splitting of the core in peripheral areas.
The application of these known laminated plates in construction uses, wherein the material is exposed to varying temperature and humidity effects, involves an additional expense for these different reasons.
In addition, sheets of wood chips and wood fibers with a matrix of aminoplastic or phenolic resins or cement or even plywood sheets are known, which have uniform density and are therefore suitable for exterior applications or inside applications where moisture is a problem. However, compared with laminated plates, these require maintenance; due to the strong absorption of water by the surface which is not sealed, they must be regularly treated on all sides, they exhibit strong swelling upon absorption of water with increases in thickness, they are not decorative and they have low stability.
A decorative, plate-shaped molded article made of wood chips has also been described (DE-OS No. 19 12 300), which contains a phenolic resin matrix, with the resin content amounting to 5 to 15% by weight relative to the dry wood chips. This molded article is likewise unsatisfactory with respect to weather resistance. It is observed that, in weatherability testing, strong absorption of water and consequent peripheral swelling and core splitting take place already after a few weeks, and as a result, cracking on the surface occurs.